Mobile crane systems often rely on combustion engines (CE) coupled to AC generators in order to supply the energy needed to operate. It is advantageous to operate such systems in the most efficient manner possible in order to minimize fuel costs and pollution. It is the nature of these systems that the power demanded of the generator varies widely over a period of time. For a very large percentage of the time, relatively small amounts of power are needed, but when the time comes to lift the load, a large surge in power is required. The generator system selected must be rated to meet the peak demand requirement. As a result, for a large percentage of the time in use, the generator system is operating much below its design rating.
The efficiency of a generator system is determined by its design and operating point. As regards design, larger systems have higher losses and higher efficiencies when losses are stated in kW and efficiencies are stated as a percentage. Once a system is selected, however, the only way to optimize efficiency is to control the operating point at which the system is operated. In any generator system, losses increase with the rotational speed of the system. Therefore, efficiency is optimized by operating the system at the lowest possible speed that will allow production of the required power.
In systems used by mobile cranes, the electrical loads attached to the AC generator system include a converter supplying a DC bus with one or more inverters attached. Each inverter in turn supplies power to the different motors of the crane system. Finally, auxiliary AC loads must be accounted for which control, for example, lights, crane cabin controls and air conditioners.
In the traditional system, the generator is operated at a constant speed in order to provide a constant voltage and frequency to the load. Having a constant speed and voltage greatly simplifies the design of the electrical system and allows it to be operated in a straight forward manner. This approach leads on the one hand to the lowest initial cost and high performance but on the other hand to lower efficiency. A schematic diagram of a one-line diagram of this type of system is shown in FIG. 1. In this system, the DC bus voltage is directly dependent on the AC bus voltage as indicated by the formula DC=AC (line-line rms)×1.35. Combustion engine 100 is coupled to generator 105 which supplies power along an AC bus 110 to auxiliary loads 115 and through diode converter 120 along a DC bus 125 to inverters 130, 135 and 140 which are connected, respectively, to loads such as hoist motor 145, gantry motor 150 and trolley motor 155.
An improvement on the traditional system for use in more complex systems is available in which the generator frequency and voltage is decoupled from the DC bus voltage. In these systems, an active converter operating solid state switches under PWM control maintains the DC bus voltage at the rated level regardless of the generator speed (and AC bus voltage). In such systems, a separate inverter is provided to supply the auxiliary loads with the constant voltage and frequency they need. This system offers improved efficiency over the traditional one but increases the initial cost by requiring the active converter to function as a separate inverter to supply auxiliary loads. A controller is included which determines the required engine speed by computing the total power required by all the connected loads. FIG. 2 provides a schematic diagram of a one-line system of this type. In this variation, combustion engine 200 is coupled to generator 205 which supplies power along AC bus 210 to active IGBT converter 215 and further supplies power along DC bus 220 to separate inverters 225, 230, 235, 240 and 245 which are connected, respectively, to loads such as resistor bank 250 which functions as a dynamic braking resistor to dissipate excess energy, hoist motor 255, gantry motor 260, trolley motor 265 and auxiliary loads 270.
What is needed is a fuel efficient crane power system with lower initial set-up and running costs and higher long-term reliability than any currently available.